This fits a model in which tetrapod ancestors carried a genetic variation that expanded the core of mesodermal tissue in their fins, which was then organized by the standard rules of limb mesoderm into bone and muscle. Again, this is opportunity, a new field of potential that in these early stages of evolution hadn’t yet been refined into a specific, and now familiar, pattern, although elements of that pattern are foreshadowed here.

Instead of “genetic variation” I would say “gene regulatory network” since the whole wiring of genes required for extra limb growth is what was already present in the common ancestor of tetrapodes and fishes. Maybe all the crazy feminism is starting to unglue him.